BackgroundPsychosocial interventions have the potential to enhance relapse prevention in bipolar disorder.AimsTo evaluate a manualised group-based intervention for people with bipolar disorder in a naturalistic setting.MethodEighty-four participants were randomised to receive the group-based intervention (a 12-week programme plus three booster sessions) or treatment as usual, and followed up with monthly telephone interviews (for 9 months post-intervention) and face-to-face interviews (at baseline, 3 months and 12 months).ResultsParticipants who received the group-based intervention were significantly less likely to have a relapse of any type and spent less time unwell. There was a reduced rate of relapse in the treatment group for pooled relapses of any type (hazard ratio 0.43, 95% CI 0.20–0.95; t343 = −2.09, P = 0.04).ConclusionsThis study suggests that the group-based intervention reduces relapse risk in bipolar disorder.